LEDs are typically constructed by growing a p-i-n diode on a substrate. The diode is constructed by growing a layer of n-doped material on the substrate, growing a light generation region on the n-doped layer, and then growing the layer of p-doped material on top of the n-doped material. An electrode is then deposited on the top surface of the p-doped layer, and a contact is made to the n-doped layer. Light may be extracted either through the substrate or through the electrode on top of the p-doped material. If the light is to be removed through the top electrode, the electrode is constructed from a transparent material such as indium tin oxide or a very thin layer of gold.
The efficiency of an LED is the product of two efficiencies, the efficiency with which power applied to the electrodes is converted to light and the efficiency with which that light is coupled out of the device. For GaN-based LEDs fabricated on sapphire substrates, a large fraction of the light generated in the diode is lost because of poor coupling efficiency. GaN has an index of refraction that is much higher than that of air or epoxy encapsulants. Accordingly, only light impinging on the surface of the diode in a small cone of angles will escape the surface. Most of the remaining light is reflected back into the GaN layer and is trapped in a waveguide bounded by the sapphire substrate bottom surface and the GaN top surface. Much of this trapped light is eventually absorbed within the device. Accordingly, the efficiency of GaN diodes is less than ideal.
One method that has been suggested for improving the extraction efficiency of an LED requires the LED to be shaped such that light generated in the device strikes the surface at the critical angle or greater, thereby preventing the internal reflection problem described above. In these LEDs, the chip is shaped as a hemisphere or truncated pyramid. Such shaping of the chip is very cumbersome and quite costly. In addition, the shapes of the surface alters the light emission profile. For example, if the top surface is in the shape of a hemisphere that is sufficiently far from the LED to assure that all light strikes the surface in the desired cone of angles, the surface will act as a lens. If the lens properties are not consistent with the product in which the LED is to be used, additional lenses must be incorporated in the product, which increases both the cost and design complexity of the product.
A second prior art method for improving the extraction efficiency utilizes a roughening of the upper surface or side surfaces of the LED by etching to destroy the planar nature of the surface thereby providing a large variety of non-planar facets through which light striking the surface can exit. While any particular facet will still allow only a fraction of the light striking it to escape, the light that is reflected back into the LED will again be reflected to the roughened surface and strike another facet whose orientation is not correlated with that of the first facet. Hence, some of this light will also escape. The light that is again reflected is recycled back to the surface and again has another chance to escape, and so on. As a result, a considerably higher fraction of the light generated in the LED is coupled out of the LED.
The prior art methods for roughening the surface involve a random etching of the top surface of the outermost crystalline layer of the LED. For example, an irregular etch pattern can be generated by depositing particles on the surface of the LED and then using the particles to define a random etch mask. The resulting pattern has at least two problems. First, the pattern can leave islands in the top electrode, which is deposited on the etched surface after the etching operation is completed. These islands are not connected to the top electrode contact through which the power connection to the electrode is made. Hence, the portion of the LED under these islands does not generate light. As a result the effective area of the LED, and hence the total light generated, is reduced.
Second, in GaN-based LEDs, the p-n junction, which contains the light generation region is placed very close to the upper surface to minimize the thickness of the p-doped material. The p-doped material has a very high resistivity and high absorption, and hence, a significant amount of power is lost if the layer is thick. If a thin p-layer is used and then etched, the etching often destroys a substantial portion of the junction. The destroyed portions do not generate light. This leads to a further reduction in the effective light generation area. If a thick layer is used, the problems associated with the high resistivity and absorption reduce the device performance.